1. Field of the Invention
The present invention relates to a method and apparatus for recording a phase change optical disc and, more particularly, to a method and apparatus for recording an optical disc at high speed exceeding recording speed which is assumed (specified) for a phase change optical disc as an object.
2. Description of the Related Art
As rewritable optical discs, phase change optical discs employed by a DVD-RAM (Digital Versatile Disc-Random Access Memory), a DVD-RW (ReWritable), DVD+RW, CD-RW (Compact Disc-ReWritable), and the like are being widely spread. In the phase change optical disc, a signal recording layer made of a rare-metal-based phase change material which reversibly changes between a crystal state (crystal phase) and a non-crystal state (amorphous phase) by heat is formed. By irradiating the signal recording layer with a laser beam, a signal is recorded.
In the phase change optical disc, an irradiated portion is changed reversibly to the amorphous state or crystal state in accordance with parameters of a laser beam emitted to the signal recording layer, and recording and regenerating is performed according to a change in the reflectance. When the signal recording layer is quickly heated by irradiation of a laser beam of the highest power level and then quickly cooled, the signal recording layer enters an amorphous state of lower reflectance, and a portion called a “mark” is recorded. On the other hand, when the signal recording layer is gradually heated by irradiation of a laser beam of a lower power level and then gradually cooled, the signal recording layer enters a crystal state of higher reflectance irrespective of the state before the irradiation, and a portion called a “space” is recorded. A signal is regenerated by using a laser beam of a power level lower than that at the time of erasing a recording signal. In this case, a recording/regenerating apparatus detects return light of a laser beam reflected by the signal recording layer, and regenerates the recording signal by using the fact that the reflectance differs according to whether the signal recording layer in the position irradiated with the laser beam is in the crystal state or the amorphous state.
Different from a magnetooptic disc, a phase change optical disc does not need external magnetic field generating means for recording/regenerating, so that overwriting or rewriting of a recording signal can be easily performed. In addition, a small number of parts is sufficient for a driving device. Thus, the phase change optical disc can contribute to reduction in the size and cost of the recording/regenerating apparatus. Since a recordable information amount is larger as compared with that of a magnetooptic disc, it is expected that the phase change optical disc will be spread further.
Usually, recording using the phase change optical disc strongly depends on speed, so that recording cannot be performed in a wide speed range, and the recording speed is limited to a certain range. To be specific, recording speed peculiar to a phase change optical disc is specified in accordance with a kind. For example, in the case of a DVD, a double-speed disc is constructed so as to be adapted to recording at a linear velocity of 6.98 m/s (meters/second), and a 2.4× disc is constructed so as to be adapted to recording at a linear velocity of 8.376 m/s. 1× as a reference corresponds to 3.49 m/s.
In the phase change optical disc, as long as recording is performed at a recording speed specified as above, a signal can be properly recorded on not only a disc in an unused (unrecorded) state but also a disc on which a signal has been already recorded by properly changing the power of a laser beam. At a specific speed, recording after erasing of an existing signal is not performed but so-called direct overwrite (DOW) of directly overwriting a disc in accordance with a modulation signal can be also performed. Therefore, it is basically possible to perform the overwriting by modulating the power of a single laser beam. However, the phase change optical disc is limited with respect to time required for crystallizing a recording material. In the case where the linear velocity is increased, time required for crystallization is short and it is difficult to perform sufficient recording.
Conventionally, a method of performing overwriting by using a plurality of beams has been also proposed. In the method, at least two laser beams of different powers for erasing and recording are used and the overwriting is carried out by allowing an erasure spot to precede a recording spot on a track. The method has an advantage such that also in the case where the linear velocity is increased to be relatively high, it is easy to assure time required for crystallization.
The method of realizing the overwriting by using a plurality of beams is disclosed in, for example, Japanese Unexamined Patent Publication No. 2002-334473. According to the method, in the case of performing overwriting, a laser beam emitted from a semiconductor laser is split into a main beam for recording and a preceding beam for erasing by a first beam splitter. By using an external modulator, the power of the preceding beam for erasing is modulated so as to attenuate from the recording power to the erasing power. The preceding beam and the main beam are combined by a second beam splitter, and the erasure spot formed by the preceding beam precede the recording spot formed by the main beam on a track of an optical disc.
However, as already described above, dependency on speed of recording is large in the phase change optical disc, peculiar recording speed at which overwriting (direct overwriting) can be performed is determined in the specification, and it is difficult to perform recording at a speed much higher than the peculiar recording speed. For example, the recording method disclosed in Japanese Unexamined Patent Publication No. 2002-334473 will be examined. The disclosed invention is based on precondition that recording is performed in a speed range which is specified (permitted) in accordance with the kind of a phase change optical disc or at a speed slightly higher than the range. The case of performing overwriting at a speed (for example, 4×=13.96 m/s) much higher than the specified speed (for example, 2× or 2.4×) is not considered. The invention does not disclose the relation between the power level of the preceding beam for erasure and the power level of the main beam for recording and a method of setting the power levels in the case of performing the recording at a speed which is not the specified speed. Consequently, even the method is applied, it is considered to be difficult to perform recording at a speed much higher than the specified speed for the following reasons.
For example, in the case of setting the level of the erasure power in the preceding beam to the normal level adapted to specific recording speed, data cannot be erased well due to insufficient power, and it is feared that the residual component (residual modulation component) appears as a jitter. When the jitter is large, it appears as an error at the time of regenerating, so that performance deteriorates. As a result, it is difficult to perform overwriting at a high speed exceeding the specific speed. A jitter is fluctuation in the time direction of a regenerated RF (Radio Frequency) signal (hereinbelow, simply called a regenerated signal) and is generally defined as a value obtained by dividing a standard deviation of the difference between an edge obtained by binarizing a regenerated signal and the timing of a reference clock extracted from the regenerated RF signal by the cycle of a reference clock.
On the other hand, the level of the erasure power in the preceding beam may be set to be higher than that of a normal power (power used in the case of specific recording speed). In this case, however, the applicant of the present invention has learned the fact that, although the modulation component of a recorded signal (RF signal) decreases as the level of the erasing power is increased, when the erasing power is increased more than a certain extent, the top level (largest value or maximum value) of the RF signal decreases. It means that when the erasing power is increased to a certain extent or higher in overwriting, the level of a portion corresponding to a space region, of the RF signal decreases and, as a result, the difference from the level of a portion corresponding to a mark region is reduced. Specifically, the degree of modulation of an RF signal (on the disc) after overwriting degrades (decreases). In the case of regenerating the signal, the C/N (Carrier-to-Noise ratio) characteristic deteriorates.
As described above, in the conventional method, in the case of overwriting a phase change optical disc at a speed much higher than the specific speed, it is difficult to suppress a jitter while sufficiently assuring the degree of modulation of a regenerated signal. That is, it is difficult to realize overwriting at a high speed which is much higher than the specific speed.